Terminals, Keyboards, and Moitors    Vladimir Zworykin and Sir William Crookes    Flat Panel Displays and Liquid Crystal Displays    Mice      Printers    Color Printers

Buses

The usual arrangement is a metal box with a large printed circuit board at the bottom called the Motherboard.  It contains the CPU chip, some slots into which DIMM modules can be clicked, various support chips, a bus etched along its length, and sockets into which the edge connectors of I/O boards can be inserted.  A single bus can be used to connect the CPU, memory, and I/O devices.  Each I/O device consists of two parts, one containing most of the electronics, called the controller, and one containing the I/O device itself, such as a disk drive.

Terminals

Computer terminals consist of two parts: a keyboard and a monitor.  They are integrated into a single device and attached to the main computer by a serial line or over a telephone line.

Keyboards

Cheaper keyboards mechanical contact when depressed.  Better ones have a sheet of elastometric material (a kind of rubber) between the keys and the underlying printed circuit board.  Under each key is a small dome that buckles when depressed far enough.  A small spot of conductive material inside the dome closes the circuit.  Some keyboards have a magnet under each key that passes through a coil when struck, thus inducing a current that can be detected.  When a key is depressed, an interrupt is generated and the keyboard interrupt handler is started.  The interrupt handler reads a hardware register inside the keyboard controller to get the number of the key ( 1 though 102) that was just depressed.  When a key is released, a second interrupt is caused.  Depressing the SHIFT key, then depressing and releasing the M key, then releasing the SHIFT  key, causes the operating system to see "M" rather than "m".  Handling of multikey sequences involving SHIFT, CTRL, and ALT, such as CTRL-ALT-DEL to reboot all IBM PCs is done entirely in software.

CRT Monitors

A monitor is a box containing a  Cathode Ray Tube  or CRT which contains a gun that can shoot an electron beam against a 
Vladimir Zworykin
phosphorescent screen near the front of the tube. This was invented by Vladimir Zworykin based on the Crookes tube invented by  Sir William Crookes   (Color monitors have three electron gums one each for red, green, and blue ).  During the horizontal scan, the beam sweeps across the screen in about 50 micro seconds, tracing out an almost horizontal line on the screen.  Then it executes a horizontal retrace to get back to the   left-hand edge in order to begin the next sweep.  This device produces an image line by line and is called a raster scanning

Horizontal sweeping is controlled by linearly increasing voltage applied to the horizontal deflection plates placed to the left and right of the electron gun.  Vertical motion is controlled by a much more slowly increasing voltage applied to the vertical deflection plates placed above and below the gun.  After somewhere between 400 and 1000 sweeps, the voltages on the horizontal and vertical deflection plates are rapidly reversed together to put the beam back in the upper left hand corner.  A full-screen image is normally repainted between 30 and 60 times a second.  Some high end models use magnetic fields rather than electric ones.  To produce a pattern of dots on the screen, a grid is present inside the CRT.  When a positive voltage is applied to the grid, the electrons are accelerated, causing the beam to hit the screen and making it glow briefly.  When a negative voltage is used, the electrons are repelled, so they do not pass through the grid, and the screen does not glow.   A binary electric signal is converted into a visual display consisting of bright and dark spots.

Flat Panel Displays

LCD  (Liquid Crystal Display)

Liquid crystals are viscous organic molecules that flow like a liquid but also have spatial structure, like a crystal.  They were discovered by an Austrian botanist, Rheinitzer in 1888 and first applied to displays (calculators, watches) in the 1960s.  When all the molecules are lined up in the same direction, the optical properties of the crystal depend on the direction and polarization of the incoming light.  Using an applied electric field, the molecular arrangement, hence the optical properties can be changed.  By shining a light through a liquid crystal, the intensity of light exiting from it can be controlled electrically.  An LCD screen consists of two parallel glass plates between which is a sealed volume containing a liquid crystal.  Transparent electrodes are attached to both plates.  A light behind the rear plate illuminates the screen from behind.  Electric fields are created.  Different parts of the screen get different voltages, to control the image displayed.  Glued to the front and rear of the screen are polaroids.

Mice

A mouse is a small plastic box.  When it is moved, a little pointer on the screen also moves.  It has one, two, three, or four buttons, used to select menu items.  Mechanical mice have two rubber wheels protruding through the bottom.  One turned while the mouse was moved parallel to the main axis, the other wen moved perpendicular.  Each wheel drove a variable resistor (potentiometer).  By measuring changes in the resistance, it was possible to see how much each wheel had moved to calculate distance in each direction.  Now a ball protrudes rather than wheels.   Another kind has a LED (Light Emitting Diode) and a photodetector on the bottom instead of wheels or a ball.   This optic mouse is used on top of a special plastic pad containing a rectangular grid of closely spaced lines.  As the mouse moves over the grid, the photodetector senses line crossings by seeing the changes in the amount flight being reflected back from the LED.  Electronics inside the mouse count the number of lines crossed in each direction.  A third kind of mouse is optomechanical.  It has a rolling ball that turns two shafts aligned at 90 degrees to each other.  The shafts are connected to encoders that have slits through which light can pass.  As the mouse moves, the shafts rotate, and light pulses strike the detectors whenever a slit comes between an LED and its detector.  The number of pulses detected is proportional to the amount of motion.

Printers

Monochrome Printers

Matrix Printer is the cheapest kind of printer.  A print head containing between 7 and 24 electromagnetically activable needles is scanned across each print line.  Low-end printers have seven needles, for printing, 80 characters in a 5 x 7 matrix across the line.  The print line consists of 7 horizontal lines, each consisting of 5 x 80 = 400 dots.  The technique can be improved by using more needles.  Multiple passes over each scan line are required to produce overlapping dots, slowing the printing rate.  They are cheap and reliable but slow, noisy and poor at graphics.  They are used for printing on large (> 30 cm) preprinted forms, on small pieces of paper, such as cash register receipts, ATM machine or credit card transaction slips, or airline boarding passes, and for printing on multipart continuous forms with carbon paper embedded between the copies.

Inkjet Printers are low-cost home printers.  The movable print head holds an ink cartridge, and is swept horizontally across the paper while ink is sprayed from its tiny nozzles.  Inside each nozzle, an ink droplet is electronically heated to the boiling point until it explodes.  The ink spurts out onto the paper.  The nozzle is then cooled and the resulting vacuum sucks in another ink droplet.  Speed depends on how fast the boil/cool cycle can be repeated.  They have resolutions of 300 dpi (dots per inch) to 1440 dpi.  They are cheap, quiet and have good quality, but are slow, use expensive ink cartridges, and produce ink-soaked output.

Laser Printers

Laser Printers have high quality images, excellent flexibility, good speed, and moderate cost.  They use the same technology as photocopy machines.  The heart of the printer is a rotating precision drum or belt.  At the start of each page cycle, it is charged up to 1000 volts and coated with a photosensitive material.  Then light from a laser is scanned along the length of the drum like a electron beam in a CRT.  A rotating octagonal mirror is used to scan the length of the drum.  The light beam is modulated to produce a pattern of light and dark spots.  The spots where the beam hits lose their electrical charge.  After a line of dots has been painted, the drum rotates a fraction of a degree to allow the next line to be painted.  The first line of dots reaches the toner, a reservoir of an electrostatically sensitive black powder, which is attracted to those dots that are still charged, forming a visual image of that line.  The toner-coated drum is pressed against the paper, transferring the black powder to the paper.  The paper is then passed through heated rollers to fuse the toner to the paper permanently, fixing the image.  The drum is discharged and scraped clean of any residual toner, preparing it for being charged and coated again for the next paper.

Color Printers

Color images can be viewed in two ways: transmitted light and reflected light.  Transmitted light images produced on CRT monitors are built up from linear superposition of the three additive primary colors, red, green, and blue.  Reflected light images, like color photographs and pictures in glossy magazines, absorb certain wavelengths of light and reflect the rest.  They are built up of the three subtractive primary colors, cyan (all red absorbed), yellow (all blue absorbed), and magenta (all green absorbed).  Every color can be produced by mixing cyan, yellow,  magenta, and black inks.  They are called CYMK Printers (K is for black, to avoid confusion with Blue).  Monitors use transmitted light and RGB system for producing colors.  Special ink and paper should be used.  Two kinds of ink exist.  Dye-based inks are colored dyes dissolved in a fluid carrier.  They give bright colors and flow easily.  But they fade when exposed to ultraviolet light , such as that contained in sunlight.  Pigment-based ink contains solid particles of pigment suspended in a fluid carrier that evaporates from the paper, leaving the pigment behind.  They do not fade, but are not as bright and the pigment particles clog the nozzles, requiring periodic cleaning.  Coated or glossy is required for printing photographs.   They are specially designed to hold the ink droplets and not let them spread out.
Solid Ink Printers accept four solid blocks of a special waxy ink which are then melted into hot ink reservoirs.  Startup times can be 10 minutes, while the ink blocks are melting.  The hot ink is sprayed onto the paper, where it solidifies and is fused with the paper by forcing it between two hard rollers.  In a color laser printer, separate C, Y, M, and K images are laid down and transferred to a roller using four different toners.  A 1200 x 1200 dpi image for a page containing 80 square inches needs 115 million pixels.  With 4 bits/pixel, the printer needs 55 MB just for the bit map, exclusive of memory for the internal processors, fonts, etc.  They are expensive, but fast, quality is high, and the images are stable over time.  Wax Printers have a wide ribbon of four-color wax that is segmented into page-size bands.  Thousands of heating elements melt the wax ass the paper moves under it.  The wax is fused to the paper in the form of pixels using the CMYK system.
 Dye Sublimation Printer .  Sublimation is the scientific name for a solid changing into a gas without passing through the liquid state.  Dry ice (frozen carbon dioxide) sublimates.  In a dye sublimation printer, a carrier containing the CMYK dyes passes over a thermal print head containing thousands of programmable heating elements.  The dyes are vaporized instantly and absorbed by a special paper.  Each heating element can produce 256 different temperatures.  The higher the temperature, the more dye is deposited, and the more intense the color.  Nearly continuous colors are possible, so no halftoning is needed.  They produce highly realistic photographic images on special, expensive paper.

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